Left clicking adds a mirror modifier
There seems to be a keymap added to your system (most likely accidentally), which calls the add modifier operator, and its default is the mirror one. Go to your User Preferences -> Input section, and search for 'modifier' in the 'Name" field:



If you find a keymap like the one in 'nicely drawn' brackets in the screenshot, remove it by clicking the X button (marked with the arrow).

More interesting is actually how you got there. I did it like this: I clicked the 'add modifier' command in the Spacebar menu. The Properties Panel one works of course as well. When the menu pops up, I RMB clicked on the Mirror Item, which brings up another menu. I chose 'Add Shortcut', and simply LMB clicked in there. And the damage was done  :)

However, it's not exactly what you have, as my version automatically adds a Mirror, it does not open the menu any more. I had to manually clear the 'Type' field in the Keybinding (see screenshot, bottom right) to have exactly your behavior. Maybe now you can remember how you achieved it? Just played with keymaps maybe?